Concerned about arc-flash
and electric shock?
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Safety technology
Introduction
Want to comply with
NFPA 70E CSA Z462?
How can IR Windows help?
Infrared thermography has become a
well-established and proven method for
inspecting live electrical equipment. To
carry out tests, the thermographer usually works with live energized equipment
and requires a clear line of sight to the
target. Thermographers must therefore
be especially aware of the hazards, the
legislation and safety issues, and the
techniques and equipment best suited
to minimizing the risks when working in
these dangerous environments.
Electrocution is the obvious danger faced by
anyone working on or near live electrical equipment and it is clearly important to understand
shock hazards and wear appropriate protection.
However, most electrical accidents are not the
result of direct electric shocks. A particularly
hazardous type of shorting fault—an arc fault—
occurs when the insulation or air separation
between high voltage conductors is compromised. Under these conditions, a plasma arc—an
“arc flash”—may form between the conductors,
unleashing a potentially explosive release of
thermal energy.
An arc flash can result in considerable
damage to equipment and serious injuries to
nearby personnel. A study carried out by the US
Department of Labor found that, during a 7-year
period, 2576 US workers died and over 32,000
suffered injuries from electrical shock and burn
injuries. 77 % of recorded electrical injuries
were due to arc flash incidents. According to
statistics compiled by CapSchell Inc (Chicago),
every day, in the US alone, there are 5-10 ten
arc flash incidents, some fatal.
NFPA 70E is the leading internationally recognized safety standard for electrical safety in the
workplace. The Canadian Standards Association
has developed its own set of standards based
on NFPA 70E: CSA Z462.
These standards define a set of safe requirements for personnel working on electrical
equipment. To comply with the standards,
employers must carry out a hazard risk assessment and ensure that all employees working in
a potential arc-flash hazard zone use appropriate equipment and wear the right protective
clothing. Although it is not the responsibility of the thermographer to put in place the
appropriate safety procedures, it is important
to recognize and understand their need, and to
ensure that the correct procedures, equipment
and protective clothing are used.
The installation of IR windows, panes or ports
allows a thermographer to inspect live electrical equipment without the removal of protective
covers and the exposure of equipment. An
arc-resistant window, unlike a port or pane,
provides additional protection for the thermographer in the event of an arc flash resulting
from unexpected component failures or work
on other parts of the system. This substantially
reduces the hazard rating for the inspections
and, in most cases, may allow the thermographer to work more safely minimizing the need
for excessively bulky and cumbersome protective clothing.
What is an arc-flash?
An electrical system can be subject to two types
of shorting faults:
• Bolted faults
• Arc faults
Bolted faults
A “bolted-fault” is everyone’s idea of a short
circuit: such as energizing the circuit with a
ground set in place. A bolted fault results in a
very high current; it is a low impedance short
because of the solid connection. Bolted faults
behave predictably and so conductors can be
rated to withstand the overcurrent for the time
required for an interrupt device to operate.
Bolted faults rarely result in an explosion.
Older switchgear which holds a “fault-rating”
will usually be rated for its ability to withstand
this high current for a particular time period. A
bolted “fault-rated” piece of equipment will usually have a BIL (Basic Impulse Level) highlighted
on the casing itself in the form of a fault current
for a set duration. E.g 100 kA for 5 seconds.
Arc faults
The second—and far more destructive—fault is
an arc-fault. This occurs when the insulation,
or more specifically the air separation, between
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electrical conductors is no longer sufficient to
withstand their potential difference. This can
occur for many reasons. A dropped tool or any
other conductive element (even rust), introduced
between or near energized components may
compromise the insulating clearances. Often,
incidents occur when a worker mistakenly fails
to ensure that equipment has been properly
de-energized. Incidents can even occur when
a worker is simply removing a cover from a
piece of equipment. A significant proportion
of arc faults occur simply due to some form of
component failure and is not limited to human
interaction alone.
In contrast to the low impedance required for
a bolted fault, an arc-fault is a high impedance
short because the discharge occurs through air.
Figure 1. Demonstration of the power of an Arc Flash.
Photo courtesy of ewbengineering.
The current flow is therefore “comparatively”
low but the explosive effects are much more
destructive and potentially lethal. Unlike a
bolted fault, it is difficult to predict exactly how
much energy will be released by an arc fault. In
particular, it is difficult to predict the duration
of an arc fault as this depends on many factors,
feedback mechanisms and the response of the
over current protection devices.
When an arc fault occurs
NIOSH (National Institute for Occupational Safety
and Health, Pittsburgh, USA) has published
the results of a survey of electrical accidents
reported by MSHA (Mine Safety and Health
Administration, US Department of Labor) in the
mining sector over the period: 1990-2001. In
more than two-thirds of the cases of arc flash
injuries, the victim was performing some form
of electrical work such as troubleshooting and
repair. More surprisingly, 19 % of the accidents
arose from the direct failure of equipment during
normal operation. Overall, 34 % of the accidents
involved some form of component failure.
The key components involved in the accidents where: circuit breakers (17 %), conductors
(16 %), non-powered hand tools (13 %), electrical meters and test leads (12 %), connectors
and plugs (11 %). Of the cases that reported the
arcing voltage, 84 % occurred with equipment
at less than 600 V and only 10 % with equipment at more than 1000 V.
19 % of the accidents arose from the
direct failure of equipment during
normal operation
Arc fault make-up
When an arc-fault is triggered, a plasma
arc—the arc flash—forms between the shorted
components. Once established, the plasma arc
has a virtually unlimited current-carrying capacity. The explosive energy release causes:
• A thermoacoustic (dynamic) pressure wave
• A high intensity flash
• A superheated ball of gas
The thermoacoustic wave is a dynamic pressure
wave caused by the instantaneous expansion of
gas local to the fault. It causes panels to rupture,
flying debris and barometric trauma. The wave
front travels outwards, away from the fault, and
as it impacts surfaces it increases in energy: an
effect known as “pressure piling”.
A common misconception is that an arc flash
will always result in panel rupture. However, by
incorporating high-speed interrupt devices and
additional protection systems, an engineer can
reduce the arc flash energy to a level where the
thermoacoustic wave front does not have sufficient energy to rupture the panel.
Although the thermoacoustic wave resulting
from an arc fault can be very destructive, it is
not the only characteristic of an arc flash. Unlike
a chemical explosion, the energy of an arc flash
converts primarily to heat and light energy.
Temperatures at the epicenter of an arc flash
can reach 20,000 °C (four times hotter than the
surface of the sun) within a millisecond. Such
high temperatures are capable of explosively
vaporizing metals such as copper. The presence
of vaporized metal can then feed and sustain
the plasma arc and exacerbate its power.
An arc flash essentially lasts until the overcurrent protective devices open the circuit. A
fast-acting fuse may open the circuit as quickly
as several milliseconds.
The consequences of an arc flash
Arc faults are potentially fatal to any personnel in the vicinity. The intense heat of the arc
flash can severely burn human skin and ignite
the clothing of anyone within several feet of
the incident. Treatment for arc flash burns can
involve years of skin grafts.
Without proper eye protection, projectiles
and molten debris can cause eye damage. The
intense UV radiation associated with the flash
can cause retinal damage. Superheated vapors
can injure lungs and impair breathing. The
thermoacoustic blast can damage hearing with
ruptured eardrums, cause collapsed lungs and
damage other internal organs. The blast can
knock personnel off their feet; falls may result in
broken bones or lead to electrocution or further
injuries on other parts of the system.
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Inevitably, a serious arc flash will damage or
even destroy the affected equipment. This leads
to extensive downtime and expensive replacement and repair.
An incident may also represent a failure on
the part of the employer to comply with industry guidelines and regulations. This could result
in a fine, litigation fees, increased insurance
costs, expensive legal actions and accident
investigations.
Standards and guidelines
The potential dangers of an arc flash can
be reduced by following the relevant safety
guidelines and using personal protective
equipment PPE.
In the USA, the following OSHA and NFPA
regulations apply to personnel working with
energized electrical equipment:
• NFPA 70 (“National Electric Code”)
• NFPA 70E (“Standard for Electrical Safety in
the Workplace”)
• OSHA Standards 29-CFR, Part 1910 (S)
1910 333
• OSHA Standards 29-CFR, Part 1926 Subpart K
• IEEE Standard 1584-2002, (“Guide for
Performing Arc Flash Hazard Calculations.”)
Many other countries have their own broadly
similar standards and regulations. For example,
Canada’s regulations can be found in CSA Z462.
In the UK, compliance with EAWR (Electricity at
Work Regulations) 1989, section 5 is required.
NFPA 70E—the safety standard
NFPA 70E defines the safe parameters for
personnel working on electrical equipment.
Although adherence is not a legal requirement, the standard provides a benchmark for
most industries to demonstrate compliance
with OSHA’s General Duty clause. An employer
adopting the guidelines offered in NFPA 70E
demonstrates a clear commitment to safe working practices and the protection of employees
from shock and arc flash hazards.
According to the standard, if personnel will be
operating in the presence of energized equipment, then certain safety considerations are
applicable. 70E recognizes that there may be
the potential for arc flash and arc blast even
when conductors are not exposed. Qualified
personnel responsible for the work must:
• Conduct an arc flash hazard analysis
• Implement qualified and general worker
safety training based on the results
• Establish shock and flash protection
boundaries
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• Provide protective clothing and personal
protective equipment to ANSI standards
• Put warning labels on equipment
• Authorize the job with a ‘live work’ permit
Steps for an arc flash hazard analysis
Section 4 of IEEE 1584-2002 outlines a 9-step
procedure for arc flash hazard analysis. The
purpose of this analysis is to “identify the flash
protection boundary and the incident energy at
assigned working distances...”
The nine steps are:
1.Collate system data. Collect system and
installation information for a detailed short
circuit assessment. You will need to describe
the system and the arrangement of its
components in a one-line drawing with
nameplate specifications for each device
and the lengths and cross-sectional areas of
interconnecting cables.
2.Consider all modes of operation. Examine
the different ways that the system operates
and how this may affect the risks and magnitudes of arc hazards.
3.Calculate bolted fault currents. Using the
data gathered in the first two steps, calculate
the highest bolted fault current expected to
flow during any short circuit.
4.Calculate arc fault currents. During an arc
fault, the current flow is normally lower than
that of a bolted fault in the same equipment
because of the added impedance of the arc.
For example, for a bolted fault of 40 kA at
480 V the corresponding arc fault would be
expected to yield about 20 kA. IEE 15842002 provides formula for estimating arc fault
currents.
5.Determine protective device characteristics and arc durations. Estimate how over
current protection devices will react during
an arc fault. These may react more slowly,
extending the duration and power of the arc
flash. Through the analysis, it may be possible to reduce the arc flash hazard and lower
the PPE requirement by replacing existing
circuit breakers. For example, modern, current-limiting fuses may considerably reduce
arc flash energies by reacting more rapidly
and at lower over current values.
6.Document system voltages. Establish bus
gaps and operating voltages.
7.Estimate working distances. Determine the
distances from arc fault sources to a worker’s
face and chest. Although hands and arms
may be closer to any incident, injuries are
unlikely to be life-threatening.
8.Determine incident energy. Calculate the
incident energy resulting from an arc fault at
the working distance.
9.Determine flash protection boundary. Use
the same calculations to estimate a ‘safe’
distance from the source of the arc hazard
beyond which PPE is not required. This paper
describes the Flash Protection Boundary in
more detail later.
Shock hazard analysis
As the name implies, the determination of the
shock hazard is an analysis designed to reduce
the risk of electrocution. NFPA 70 recommends
the identification of three boundaries to define
the safe working limits for personnel working in
an area with shock hazards. Each area is associated with a level of training and PPE.
NFPA 70E data (contained in Table 130.7(C)(2)
allows you to calculate the boundaries using a
formula based on the voltage of the equipment.
The limited approach boundary
This is the minimum permitted distance that
unqualified and unprotected personnel may
approach a live component. Before crossing the
limited approach boundary and entering the
limited space, a suitably qualified person must
use the appropriate PPE and be trained to perform the required work. An unqualified person
may enter the limited approach area if they are
under the supervision of a qualified person.
The restricted boundary
To cross the restricted boundary and access
the restricted space, personnel need to have
been trained in shock protection techniques, be
wearing the correct PPE and have a written and
approved plan for any work in the zone. The
plan must make it clear that the worker must
not enter the prohibited space or cross the prohibited boundary either personally or by using
any equipment or tool.
Prohibited boundary
No worker should cross the prohibited boundary
and enter the prohibited area unless:
• The responsible authority has carried out a
full risk assessment.
• The work has been documented and it has
been fully established why it must be carried
it out on live equipment.
• The qualified worker has been trained to
work on live electrical equipment.
• The worker has been equipped with appropriate PPE. In terms of safety, any worker
crossing the boundary must be equipped and
protected as they would be for making direct
contact with the exposed live equipment.
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Figure 2. The shock hazard boundaries
Establishing these boundaries is an important
step in protecting staff from the dangers of
electrocution. It ensures that personnel use the
correct equipment and procedures when in the
proximity of live electrical equipment.
However, if the live equipment also poses an
arc flash hazard, it is important to establish a
‘safe’ distance for this eventuality: the arc flash
protection boundary.
The flash protection boundary
The arc flash protection boundary (FPB) is the
minimum ‘safe’ distance from energized equipment that has a potential for an arc fault. It is
defined as the distance at which, in the event
of an arc flash, a worker would be exposed to a
thermal event with incident energy of 1.2 cal/
cm² for 0.1 second. With this exposure, a worker
may receive a 2nd degree burn to exposed skin.
If it is necessary for workers to cross the flash
protection boundary, and potentially be exposed
to higher incident energies from any arc flash,
they must be wearing appropriate PPE. For
example, at an incident energy greater than
1.2 cal/cm², clothing could ignite and bare skin
would sustain 2nd degree burns.
An important point here is that the flash
protection boundary and the rules governing access within it take precedence over the
shock hazard boundaries. So, for example, if
the flash protection boundary is greater than the
limited approach boundary then no unqualified
person can be permitted in the limited approach
area and even qualified workers must wear
appropriate arc-resistant PPE here.
You can determine the flash protection boundary for an electrical system using the calculating
methods contained in NFPA 70E and IEEE Std
1584. The equations are based on the voltage
level, fault level and the trip time of the protective device.
The conditional flash protection boundary
is 48 inches for low voltage (<600 V) systems
where the total fault exposure is less than 100k
amperes-seconds (fault current in amperes multiplied by the upstream device clearing time in
seconds). On such a system, a qualified person
who works closer than 48 inches from the live
components must wear PPE for arc-flash protection including flame-resistant (FR) clothing. Of
course, further PPE may be necessary for protection against electric shock according to the
location of the shock protection boundaries.
Please refer to IEEE 1584 for comprehensive
calculation methods for a wide range of electrical systems; the procedures describe calculation
methods for equipment with voltages in the
range: 208 V to 15 kV.
Figure 3. The Flash protection boundary takes precedence over the
shock hazard boundaries.
Choosing the right PPE
As an option to incident energy analysis to
assist in the choice of appropriate personal
protection equipment for arc flash hazards, NFPA
70E defines five hazard risk categories (HRCs):
0, 1, 2, 3, and 4.
Hazard/Risk
Category
0
1
2
3
4
APTV rating
(cal/cm²)
N/A
4
8
25
40
Figure 4. Category 4 PPE.
Copyright image courtesy of Salisbury by Honeywell
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What is appropriate PPE?
Clearly, the potential injurious effects of an
arc flash can be reduced by using a fire flame
resistant (FR) suit of a suitable calorie rating
to reduce the indecent energy on the body to
an extent that the burns suffered are not life
threatening.
Most garments are tested and rated for incident
radiation at a distance of either 18 inches or
24 inches. This roughly corresponds with the
position of the head and chest when working directly on equipment. Of course, during
an arc flash incident, the hands and arms may
be much closer to the arc fault source and may
need protective equipment with a considerably
higher rating.
While NFPA 70E and IEEE 1584 cover the
PPE requirements for arc flash protection, there
are also other considerations and standards to
include in any safety appraisal. Workers may
require eye protection, insulating gloves, ear
and hearing protection, head impact protection
and reinforced footwear.
Figure 5. Examples of PPE for arc flash protection.
Copyright image courtesy of Salisbury by Honeywell
While safety is the paramount concern, it is
important to select PPE appropriate for the task.
It might seem a good idea to insist on category
4 PPE for all live work, perhaps to avoid a timeconsuming arc flash hazard analysis, and this
may outwardly appear to be a ‘safe’ policy for
personnel. However, the use of restrictive or
excessive PPE can also be hazardous: an overheating worker struggling with poor visibility
and restricted movement is more likely to have
an accident. More accidents, more downtime.
In addition, there are scenarios where HRC
may not be enough arc-flash protection. The
HRC does not account for arc-blast.
NFPA 70E provides several tables listing the
PPE appropriate for work within the flash protection boundary: clothing and equipment such
as gloves, hats or hoods.
In addition to the HRC classification, PPE is
often described by the arc thermal performance
value (APTV). This corresponds to the capability
of the garment to withstand a particular incident energy (in cal/cm²).
As we have already discussed, the flash
protection boundary defines the distance where
an arc flash would produce incident energy of
1.2cal/cm²: a level at which 2nd degree burns
could occur. This corresponds to risk category
0. In practice, a worker will need to approach
the electrical system much closer than the flash
protection boundary. It is therefore important
to calculate the likely incident energy for the
working position and select PPE accordingly.
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Heeding the warning signs
Equipment such as “Switchboards, panelboards, industrial control panels, meter socket
enclosures, and motor control centers in other
than dwelling occupancies, which are likely
to require examination, adjustment, servicing,
or maintenance while energized, shall be field
marked to warn qualified persons of potential
electric arc flash hazards. The marking shall be
located so as to be clearly visible to qualified
persons before examination, adjustment, servicing, or maintenance of the equipment.”
Although the basic requirement is for a sign
warning of the arc flash hazard (see Figure 6) it
is more helpful for engineers if the sign includes
other useful information such as:
• Operational voltage
• Fault current
• Flash hazard boundary
• Incident energy at the normal working distance for the arc fault hazard.
• If this information is not present on the
warning sign, it should be documented and
accessible to all relevant personnel.
It is good working practice to make labeling
clear and accurate; too much information is as
bad as too little. If a system has several access
points and arc flash hazards, label it with the
parameters for the greatest hazard.
online and on load coupled with the fact that
IR Cameras cannot “see” through panel covers,
thermographers face severe safety hazards
when attempting to scan electrical distribution
equipment.
Taking measurements with a cover removed is
not a safe option.
The ideal solution is the provision of a permanent “access point” in the equipment housing
Leaving the cover closed:1.Increases safety.
2.Reduces the background reflection quotient
and reduces the interference making readings
more repeatable.
The safer alternative is the provision of a
permanent “access point” in the equipment
housing.
Figure 6. Energy level of 201 cal/cm² @7.2K V
Ports, panes and windows
The “live work” permit
Finally, before any work can commence, the
responsible manager must generate and sign an
energized electrical work permit. This describes
the task and why it must be performed with live
energized equipment, long with the shock and
flash boundaries plus related PPE.
Arc flash hazards and the electrical
thermographer
The goal of any electrical thermographer is to
prevent unplanned shutdowns in a manner that
is reliable, repeatable and above all safe. Since
thermographers work with the target equipment
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For infrared thermography, there are three types
of access point:
• Infrared Port
• Infrared Pane
• Infrared Window
An IR port is simply a hole or series of holes; an
IR pane is a thin polymer optic. Both limit physical access (human contact) to live equipment.
However, in the event of an arc fault neither
option provides a protective barrier between the
thermographer and the exposed conductor and
arc flash source. A pane affords little protection
since it is a thin polymer with a low melting
point.
How can infrared windows help?
An infrared window provides a solid barrier
between the thermographer and the live conductors. By careful design, it is possible to not
only to reduce the trigger effects of an arc but
also provide the thermographer with a far safer
working environment.
Thermography windows are relatively new
technology, so there is no specific standard that
relates to construction and testing. However,
since they are invariably installed close to arc
flash hazards, it is important that windows can
withstand not only an arc flash incident but also
the rigors of their environment and normal dayto-day operation.
In the United States, Canada and Europe,
there are differing standards associated with
testing equipment to be deemed “arc-resistant”,
specifically: ANSI C37.30.7 (North America),
EEMACS G14-1 (Canada), IEC62271 (Europe).
These standards are manifestly different and
cannot be translated from one to another.
For example, a product tested to IEC62271 in
Europe cannot be claimed to be suitable for
North American ANSI C37.20.7 nor Canadian
EEMACS G14-1.
End-use requirements vary considerably so
testing and certification needs to be generic in
order to provide an all-round product suitable
for use in most if not all applications.
Manufacturers tend to design viewing panes
to withstand accidental impacts from untrained
personnel in transit rather than the combined pressure piling and sudden temperature
increase effects of an internal electric arc.
Infrared windows, on the other hand, are constructed of crystal optic materials designed to
better protect the infrared thermographer under
an arc-flash condition during scheduled, periodic inspection of the internal equipment.
Inspection devices may also feature locking
security covers. This ensures only a trained and
authorized person can remove and complete
an inspection or scan. It also protects the optic
material from day-to-day impacts and offers
further arc-flash protection. Properly constructed
cover designs are manufactured from materials
that offer substantially similar properties as the
original panel wall knock-out—National Electric
Code 110.12(A).
Selecting the correct material
There are numerous materials that can “transmit” infrared radiation from low cost thin film
plastics used for home intruder alarm systems to
germanium optics for military imaging.
Unfortunately, there are not many materials
suitable for the task of permanent installation
into electrical equipment due to the combined
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temperatures and pressure experienced during
an arc-flash.
Typically, infrared windows are manufactured
from a crystal optic material that allows infrared and visual inspection via the same product.
This material choice, if designed and implemented correctly, can withstand an electric
arc and provide a measure of protection to the
thermographer.
Conversely, thin film polymers can transmit IR
in certain wavelengths—although actual transmission is poor—however the polymer itself
cannot withstand the temperature and pressure
of an electric arc and hence could become a
dangerous molten projectile.
The most widely used optic material is that of
crystal. A properly coated crystal optic can:
1.Maintain IR camera flexibility
2.Allow visual inspection
3.Enable corona inspection
4.Be arc-resistant
Understanding transmission
Infrared window transmission is a commonly
misused term when it comes to obtaining a
measurement. Since no material is 100 %
transmissive, other factors come into play when
attempting to correct for the apparent error.
As any good thermographer knows from
training:
Reflection + Absorption + Transmission = 1
A thermographer must think of the window and
thermal imager as an integrated system.
A little known fact is that the spectral range of
an infrared camera varies from one model to the
next. This is down to the individual “Switch-on,
Switch-off” parameter of the infrared detector.
For example, one longwave camera may have a
working spectral response of 8.1 µm to 13.9 µm;
the next unit to leave the line may operate at
7.9 µm to 13.5 µm. When this differing spectral
response on the detector is mapped against an
IR window product, the apparent “transmission”
changes as a function of the detector/window
relationship.
With a crystal window, this relationship is
relatively straightforward to understand as the
“route” through the optic is consistent. However,
when mesh is introduced into the equation,
the relationship becomes more complex still.
The route through the combined polymer/mesh
optic is confused and inconsistent resulting in a
vignette problem (a vignette effect is known to
photographers as the way a photograph fades
towards its edges—often used for effect, it can
be an unintentional result of the optical limitations of the camera’s lens). Even obtaining a
good image consistently is a challenge when
attempting to scan through mesh.
Crystal windows
With a crystal infrared window, the R + A + T
= 1 effects are multiplied with respect to the
camera by a factor of two.
These additional signals can be simplified
and compensated for by making some practical
assumptions based on real life applications.
The first point to understand when investigating transmission is that the apparent error in
the camera will not always be a negative, i.e.
the camera may read LOW or it may read HIGH
depending upon contributing factors.
Figure 9 shows an infrared camera scanning
through a crystal window at a target. Let us
assume that the emissivity of the target is 1.
The camera shows a positive error: it is reading
higher than the actual target temperature.
Figure 10 shows the same infrared camera
scanning through the crystal window at the
same target. This time however, the optic temperature is at the ambient temperature of 30 °C
The camera shows a negative error: it is reading lower than the actual target temperature.
At first glance, these two examples seem to
indicate that the error associated with electrical
inspection via a window cannot be compensated for:
• If the optic temperature is equal to or lower
than the target temperature, the camera will
read low.
• If the optic temperature is higher than the
target temperature, the camera will read high.
However, by thinking in more detail about the
application, we can make sensible assumptions
that remove the latter.
Since unpowered switchgear is sitting at
ambient temperature, the whole of the body and
internals—including the window optic—is also
at ambient temperature. When the switchgear
is energized, current begins to flow through the
internal conductors. This increases the temperature of potential targets above that of the
original ambient temperature, leaving the casing
(and window optic) at ambient.
Since scanning de-energized equipment will
not provide meaningful results, it is safe to
assume that the outer casing and hence the
window optic is at a similar temperature or
lower than that of the target.
Combined polymer and mesh windows
With a combination product such as a mesh/
polymer optic, the mesh and polymer components have differing absorption and reflection
coefficients. In this case, we can never account,
in a repeatable manner, for the infrared radiation incident on the detector. The reflected
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Figure 8. Additional signals resulting from the use of a window.
Panel
Target
Optic
Figure 9. An infrared camera scanning through a crystal window at a
target.
Target Temperature = 50 °C (122 °F)
Ambient Temperature = 30 °C (86 °F)
Window Optic Temperature = 60 °C (140 °F)
Indicated Temperature = 55 °C (131 °F)
Panel
Target
Optic
Figure 10. An example of negative error.
Target Temperature = 50 °C (122 °F)
Ambient Temperature = 30 °C (86 °F)
Window Optic Temperature = 30 °C (86 °F)
Indicated Temperature = 45 °C (113 °F)
quotient is multiplied many times over because
of the mesh and polymer material reflectivity
difference.
Another problem with scanning through mesh
is the deflection angle. This term describes the
inconsistent effect of the mesh with respect
to reflected infrared radiation. Similar to the
faceted faces used on modern stealth aircraft,
combined mesh and polymer optics deflect
infrared radiation in different paths that may or
may not be detected by an IR camera.
These confusing effects are increased further
when mesh is used on both sides of a polymer optic. The radiation from the target is not
transmitted in a repeatable manner through
the IR window; instead, the radiation is in
part deflected back into the panel and in part
transmitted through the first mesh, only to be
absorbed by polymer and the remaining energy
deflected again by the outer mesh.
The resulting signal registered by the camera
may allow a thermographer to locate a hotspot
BUT the transmission of the optic cannot be
accounted for and modelled repeatably.
In order to be sure that an IR Window installation does not lower a panel’s environmental
integrity, the original NEMA classification of the
host equipment must be matched or exceeded
by the NEMA/UL50 Type classification of the
component. Reputable manufacturers will provide third party certification of the type rating of
the component, to prove due diligence.
Window “Standards and codes”
An infrared window is first and foremost an
industrial electrical component: electrical design
and testing parameters must be applied.
Within North America, an electrical component
must be recognized by a Nationally Recognized
Testing Laboratory (NRTL) in order for that component to be acceptable to OSHA. Underwriters
Laboratories (UL) are probably the most widely
known NRTL although there are others.
UL have standards and codes that apply to
differing electrical components and infrared
windows are no different. Some of the requirements are material specific whilst others are
functionality based.
For example, UL746C is a material standard
that must be adhered to if a product destined
for use as part of electrical equipment contains
polymers. If the product has no polymers used
in its construction, then UL746C does not apply.
UL50, by comparison, is a functional test
that is denoted by a type number and is harmonized to the NEMA environmental test. In
order to obtain a simple UL50 Type 1 (Indoor)
certification, a product must show that it is
manufactured from components that are noncorrosive. There are no environmental tests such
as hose-down or dust applied to type 1 components. If a component is designed for outdoor
equipment, it will have a UL50 Type rating
higher than Type 1. A typical outdoor rating is
Type 3/12. To achieve Type 3/12, a component is subjected to a series of vigorous tests
to ensure that the sealing system and general
construction are sufficient to maintain the environmental integrity of the component when put
into service outdoors.
It is not possible to derive a North American
(NEMA) type rating from European (IP) rating.
Terms such as “equivalent to IP65” or “selfcertified” are often used by other manufacturers
whose product cannot pass the more stringent
North American environmental testing.
11
Fluke Corporation
Concerned about arc-flash and electric shock?
Arc-resistant windows
The strength of crystal infrared window optics
has been increased to such an extent that they
can withstand the effects of an arc fault.
With the adoption of NFPA 70E and the
industry’s focus on arc-flash safety, installing a
product that can withstand arc-flash should be
a primary concern for any end-user.
Finally, some arc flash myths
It is important to separate the truth from the
myth:
• “99.99 % of arc-flash events occur with the
cover removed”.
This statement is untrue: as mentioned earlier,
19 % of failures occur because of component failures with equipment during normal
operation.
A simple industry pointer would be the manufacture of arc-resistant switchgear. If arc-flash
events only occurred with open covers, there
would be no market for arc-resistant switchgear: it is only “arc-resistant” with the covers
closed.
It is imperative that an infrared window product is designed to withstand the pressure and
temperature of an arc-fault.
Conclusions
An electrical thermographer must work closely
with live energized equipment and be aware of
the dangers of this environment.
In addition to the obvious hazard of electrocution, workers must be particularly aware of the
dangers of arc flash and arc blast events. Up to
77 % of all electrical injuries are caused by arc
flash incidents. It is important to remember that
NFPA 70E does not protect personnel against
the effects of arc blast.
An arc flash is an explosive discharge resulting from a compromise of the insulation
between two conductors or a conductor and
ground. The event is characterized by its high
temperature plasma and this can cause serious burns and other injuries to an unprotected
worker several feet from the equipment.
NFPA 70E is the leading internationally recognized safety standard for arc flash prevention
and protection. The guidelines recommend a
thorough arc flash hazard analysis to establish
the nature and magnitude of the hazard, calculate the shock and flash protection boundaries,
and identify the appropriate protective clothing
and personal protective equipment required for
‘Live’ work. Warning labels on the equipment
must identify the hazard and summarize this
information.
The use of windows can limit the exposure
of a thermographer to energized equipment,
reduce the hazards of both electrocution and arc
flash and significantly reduce the need for bulky
PPE. It is important that inspection equipment is
constructed from suitable arc-resistant materials although there is currently no internationally
recognized standard covering their manufacture.
For electrical inspections via a crystal
window, the indicated temperature reading on
the camera will almost certainly be lower than
that of that target. Due to the consistent manner
associated with the error, quantitative measurements are possible as long as the camera and
crystal window are paired. This is generally a
one-time requirement as window transmission
is consistent for each model.
Conversely, a mesh/polymer optic cannot
be used for quantitative inspection because of
inconsistent transmission. This results from the
mesh deflection angle and the differing reflectivity of the mesh material and polymer optic
material. The thermographer may be able to
correct for transmission in a lab using a onetime set of parameters but the poor performance
of the mesh/polymer and lack of repeatability
means that the this optic solution cannot be corrected for in a manner that is acceptable.
When selecting an IR Window, window or
port an end-user must consider the electrical implications of the installation and insist
on third party certification to back up any
manufacturer claims. Potential mistakes such as
installing a Type 1 component into a NEMA 4
panel could cost thousands of dollars in repairs
due to leakage and equipment failure.
Finally, an optic material that may melt during
an arc flash and cause contact burns could be
more hazardous to a thermographer than simply
having the panel open for the measurements. A
true arc-tested optic should be used in order to
demonstrate due diligence.
For more information please visit www.fluke.
com/irwindows or call 1-800-760-4523.
Further reading
NFPA 70-2008. National Electrical Code.
NFPA 70E-2009. Standard for Electrical Safety in the Workplace.
IEEE Standard. 1584-2002. IEEE Guide for Performing Arc-Flash
Hazard Calculations.
CSA Z462. Standard on Workplace Electrical Safety, Canadian
Standards Association
“Protecting Miners from Electrical Arcing Injury” James C. Cawley,
P.E., and Gerald T. Homce, P.E.; NIOSHTIC-2 No. 20032718,
National Institute for Occupational Safety and Health.
EWG Engineering, LLC (Electrical Engineers), www.ewbengineering.com
Fluke. Not just infrared.
Infrared you can use.™
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
In Europe/M-East/Africa +31 (0) 40 2675 200 or
Fax +31 (0) 40 2675 222
In Canada (800)-36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.fluke.com
©2010 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 9/2010 3527077C A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
12
Fluke Corporation
Concerned about arc-flash and electric shock?